202 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1949 
without stopping and to maintain accurate time; with a battery of 
such clocks, all controlled by the same atomic vibration, it would be 
possible to bridge over the stoppage of any single clock and thereby 
to maintain an accurate standard of time more or less indefinitely. 
There will be definite objections to using as the fundamental unit of 
time a unit that is known to be variable. A new unit should be 
absolutely invariable. A clock based fundamentally upon a length 
which is controlled by an atomic wave length, and upon the velocity 
of light, for instance, seems theoretically ideal. 
Note ‘ADDED IN PROOF.—Since the lecture was delivered, investi- 
gations at the Greenwich Observatory have established the existence 
of a fairly regular annual variation in the rate of rotation of the 
earth. Relative to uniform time the earth gets behind by about 60 
milliseconds in May-June and ahead by a similar amount in Novem- 
ber. The corresponding variations in the length of the day amount 
to somewhat more than 1 millisecond a day on either side of the 
mean value. H.S. J. 
